FIG. 1 shows an example of radiation of electromagnetic waves from electronic components mounted on an electronic circuit board in which a circuit board has been printed (hereinbelow referred to as “printed board”).
FIG. 1 shows a state in which electromagnetic waves 4 are radiated from LSI (Large-Scale Integration) 2 as the electronic component mounted on printed circuit board 1 and a state in which electromagnetic waves 4 are radiated from wiring 3 that joins LSI 2.
To prevent electromagnetic wave interference that includes generation of the above-described electromagnetic wave radiation, a method known as shielding is used in which circuits or components that radiate electromagnetic waves are enclosed by a metal material. Patent Document 1 (JP-A-2006-237036) discloses a construction in which components are each separately shielded by a flexible board on which shield wiring composed of mesh wiring is provided.
FIG. 2 shows an example in which only component 2, that radiates electromagnetic waves, and circuit parts in the vicinity of component 2 are enclosed by shield part 5.
The four sides of shield part 5 that contact the printed board are brought into a conductive state with a ground pattern on printed board 1. When only component 2 and the circuit parts in its vicinity are enclosed by shield part 5 and printed wiring 3 for signals, or when small parts 14 are arranged at parts of shield part 5 that contacts printed board 1, as shown in FIG. 2, cutouts 6 must be provided in shield part 5 to avoid these parts.
The principles of the effect realized by a shield that encloses only components that radiate electromagnetic waves are next explained using the sectional view of FIG. 3.
Due to the electric field and magnetic field of electromagnetic waves 4 radiated from the surface of LSI 2, current 7 is generated in the surface of shield part 5. Due to the skin effect of shield part 5 that has a fixed thickness, current 7 only flows over the surface of the other side that is irradiated by electromagnetic wave 4, returns by way of the ground pattern on printed board 1 to LSI 2, and cancels energy.
When there are cutouts 6 in shield part 5 as shown in FIG. 2, current 7 that flows through shield part 5 may pass through the end surface of shield part 5 by the path shown by arrow 7 and return to LSI 2.
When current returns by the path shown by arrow 7 in FIG. 4, a slit antenna is formed by the current in the reverse direction that flows through shield part 5 that is a conductor and the ground pattern of printed board 1 and insulators between these currents (in this case, air).
Normally, wavelength λ of a particular frequency F (MHz) is found by the equation λ(m)=(300/F)×contraction rate. In order for metal to function effectively as an antenna, the metal must have a fixed length with respect to the wavelength in order that a harmonic current such as λ/4 or λ/2, depending on the conditions of its arrangement, resonate in the conductor, but it is known that a slit antenna functions as an effective antenna even when the length of slits (cutouts) is sufficiently short with the respect to wavelength. As a result, the cutouts may function as an effective slit antenna and electromagnetic wave radiation may be generated.
One shield method consists of enclosing the entire printed board.
In the case of complete sealing, and moreover, in the case of a metal material having a thickness of at least the depth to which harmonic current enters due to the skin effect, harmonic current flows only inside the shield component, and radiation is therefore not generated outside the shield.
However, because securing a printed board within a shield component of this construction is difficult, separate shield materials are typically combined to realize the shield component. In such cases, a portion of the harmonic current that flows through the metal surface flows through the gaps of the fitting parts of each shield material, and this portion reaches the surface of the shield component.
When using shield material of a size that would enclose a printed board, the shield material functions as a high-efficiency antenna such as the previously described ½-wavelength or ¼-wavelength antenna with the current that flows through fitting parts as the excitation source and electromagnetic wave radiation is generated.    Patent Document 1: JP-A-2006-237036